5.12 merge window, part 2

Architecture-specific

• The RISC-V architecture has gained support for non-uniform memory access (NUMA) systems. This architecture also now supports kprobes, uprobes, and per-task stack canaries.

Core kernel

• The kcmp() system call can now be configured into the system independently of the checkpoint/restore functionality.

Filesystems and block I/O

• ID mapping for mounted filesystems, which has seen several proposed implementations over the years, has been merged at last. See this merge message for more information. This functionality is currently supported by the FAT, ext4, and XFS filesystems.
• The NFS client implementation has gained support for "eager writes". When this option is enabled (at mount time), file writes are sent immediately to the server rather than sitting in the page cache. This can reduce memory pressure on the client, provide immediate notification if the filesystem fills up, and can even evidently improve throughput for some workloads.
• The CIFS ("SMB") filesystem has a couple of new mount options to control the caching of file (acregmax) and directory (acdirmax) metadata.

Hardware support

• Miscellaneous: Playstation DualSense gamepads and force-feedback game controllers, Nintendo 64 game controllers, Nintendo 64 data cartridges, Intel Lightning Mountain centralized DMA controllers, Compute Express Link 2.0 type-3 memory devices, Broadcom VK accelerators, Qualcomm MSM8939 and SDX55 interconnect buses, Microchip AXI PCIe host bridges, Intel LGM SSO LED controllers, and Canaan Kendryte K210 reset controllers, pin control units, and clock controllers.
• Pin control: R-Car V3U pin controllers, Allwinner H616 pin controllers, and Qualcomm SM8350 and SC8180x pin controllers.

Miscellaneous

• The user-space perf-events tools have gained a number of new features, including the ability to report on instruction latency and a new daemon mode for long-running sessions. See this merge changelog for more information.

Virtualization and containers

• Support for the ACRN hypervisor has been added.

Internal kernel changes

• The build system now can use Clang's link-time optimization (LTO) features on the Arm64 and x86 architectures. Android builds have been using LTO for a while, but now this capability is in the mainline as well. See this commit and this commit for (some) more information.
• The EXPORT_UNUSED_SYMBOL() and EXPORT_SYMBOL_GPL_FUTURE() macros have been removed, since there have been no users of them in the kernel for years.
• A new memory-debugging tool called "kfence" has been merged; it can find a number of types of errors (use-after-free, buffer overflow, etc.) and features a low-enough overhead, it seems, to be usable on production systems. See this documentation commit for more information.
• The core of the io_uring subsystem has been reworked to stop using kernel threads; instead, when work must be done in thread context, a new thread is forked from the calling process. This should result in no user-visible changes other than, it is hoped, a reduction in bugs from the removal of some problematic kernel code.

The 5.12 kernel has now entered the stabilization part of the development cycle. Unless something surprising happens, the final 5.12 release can be expected on April 18 or 25. Given that, seemingly, even record-breaking winter storms are unable to slow down the pace of kernel development, that something would have to be surprising indeed.